The optical fiber has found widespread use because of its extraordinary ability to guide light over considerable distances with little attenuation. This ability is typically maintained over a sizable spectral range, a feature which in some applications becomes troublesome by allowing guidance of deleterious light.
A first example of such an application is the generation of short pulses of light with Q-switched fiber lasers. As known in the art, amplified spontaneous emission (ASE) constitutes an important limiting factor in such lasers [M. Morin et al., Q-switched Fiber Lasers, Chapter 7 in Rare-Earth-Doped Fiber Lasers and Amplifiers, Second Edition, Revised and Expanded, M. J. F. Digonnet ed., Marcel Dekker, 341-394 (2001)]. The operation of a Q-switched laser involves a pumping stage during which laser oscillation is impeded by strong intra-cavity optical losses. The objective of the pumping stage is to store as much energy as possible within the gain medium, thus realizing as well the highest possible gain. The intra-cavity losses are then turned off suddenly, either by external means (active Q-switching) or by the optical power itself (passive Q-switching). The high gain coupled to the low cavity losses ensures a very rapid build-up of the laser oscillation leading to the formation of a short and energetic optical pulse. The ASE taking place during the pumping stage can lead to a sizable depletion of the energy stored in the gain medium, thus lowering the available energy and the gain at the time the intra-cavity losses are turned off. This leads to weaker and longer pulses. Wideband intra-cavity attenuation of the ASE is therefore desirable to keep it from reaching appreciable levels.
Amplified spontaneous emission can be troublesome in cw (continuous wave) fiber lasers as well [Y. Glick et al., Single Mode 1018 nm fiber laser with power of 230W, Proc. SPIE 9728, 97282T (2015)]. From a thermal management point of view, it is preferable to run a high-power fiber laser at the shortest wavelength afforded by the gain medium. This reduces the Stokes defect between pump and laser photons, i.e. the difference between the energy of each pump photon absorbed to excite the gain medium and that of each photon emitted by the laser. Globally, running the laser at a shorter wavelength reduces the difference between the pump power absorbed by the gain medium and the optical power emitted by the laser. This difference comes out as heat that must be dissipated somehow. Operation at a short wavelength is difficult when the available gain at longer wavelengths is stronger. Narrow band reflectors, that provide feedback at the shorter wavelengths but not at longer wavelengths, can be used to avoid lasing at the longer wavelengths. Even in this case, the stronger gain at longer wavelengths can lead to a powerful emission of ASE and a sizable reduction of the laser emission at the shorter wavelength. Such lasers can thus benefit from the intra-cavity wideband filtering of ASE as well.
The optical fiber, by allowing the propagation of intense light over long distances, is an ideal medium for observing nonlinear effects. One of these effects is Raman scattering, resulting from the interaction between an intense optical field and the glass molecules constituting the fiber [G. P. Agrawal, Nonlinear Fiber Optics 2nd ed., Academic Press, Chapter 8, 316-369 (1995)]. Raman scattering manifests itself as a transfer of power from an incoming optical wavelength to a longer wavelength, the spectral shift being characteristic of the material where it occurs. In fused silica, the Raman gain extends over tens of nanometers and is maximum at a wavelength shift of 46 nm when the incident light has a wavelength of 1000 nm. Raman scattering can be a serious impediment in various applications. It limits the reach of optical fiber communication links that can be achieved by increasing the optical power of the signal launched in the fiber. When the optical power reaches a threshold value (see e.g. G. P. Agrawal, Nonlinear Fiber Optics 2nd ed. supra, section 8.1.2), Raman scattering sets in and leads to a sizable transfer of power to longer wavelengths [J.-P. Blondel et al., Elimination of optical power limitation due to stimulated Raman scattering in fiber optic links, U.S. Pat. No. 6,529,672]. This is clearly problematic in optical communications links where each channel is carried by a given wavelength. Raman scattering is significant when watt-level optical powers propagate over kilometers of single mode fiber. In fiber laser systems operating at kilowatt-level optical powers, Raman scattering sets in over commensurably shorter fibers. It contributes to a detrimental spectral widening of the laser output beam [T. Schreiber et al., Analysis of stimulated Raman scattering in cw kW fiber oscillators, Proc. SPIE 8961, 89611T (2014)]. The main application of high power fiber lasers is material processing. A high-power fiber laser and the optical link carrying the laser output are designed to get the laser output to a target with minimum losses. One motivation is to maximize the efficiency of the material processing. Another is reliability and security, ensuring that the high optical power does not go where it should not. Light generated by Raman scattering, being at a sizably different wavelength than that generated by the laser gain medium, can interact differently than designed for with mirrors, filters, optical coatings and optics, reducing efficiency and raising reliability and security concerns. Raman scattering can take place in the fiber laser itself but also in optical fiber links coupled to the laser. Given the high optical powers involved, the light generated by Raman scattering can become quite powerful. Back reflection of this powerful Raman light in the fiber laser can destabilize its operation and even lead to optical damage [V. P. Gapontsev et al., Method and apparatus for preventing distortion of powerful fiber-laser systems by backreflected signals, U.S. Pat. No. 7,912,099]. Raman scattering is a major impediment limiting the achievable power in fiber laser systems.
Diverse types of fibers have been proposed to either thwart the generation of wideband deleterious light or attenuate preferentially wideband deleterious light. By way of example, in the case of Raman scattering and other nonlinear effects, one approach is to increase the transversal extent over which light is carried by an optical fiber, thus reducing the optical intensity (W/cm2) for a given optical power (W). A vast body of technical literature is devoted to large mode area (LMA) fibers, i.e. fibers with a transversal structure that supports a larger fundamental core mode (see e.g. [J. M. Fini, Large-mode-area optical fibers with reduced bend distortion, U.S. Pat. No. 7,783,149] and references found therein). However, there is a practical limit to this approach as the sensitivity of a fiber to bending typically increases with the fundamental mode effective area. Still larger core fibers can be used that support multiple core modes, but at the expense of a reduction in the optical quality of light carried by the fiber, which is then more difficult to focus to a tight spot. Coiling slightly multimode fibers can be used to attenuate preferentially higher order modes [Selecting the Optimal LMA Fiber, Application Note NuAPP-2, Nufern]. Various optical fibers have also been proposed that provide preferential attenuation over specific wavelength bands (see e.g. S. G. Grubb et al., Optical fiber gain medium with evanescent filtering, U.S. Pat. No. 6,118,575; R. T. Bise et al., Optical fiber for suppression of amplified spontaneous emission, U.S. Pat. No. 7,272,287; T. Tam and J. C. Knight, Optical power delivery system, U.S. Pat. No. 7,643,715; R. Goto, Photonic bandgap fiber, U.S. Pat. No. 8,035,891; A. Petersson et al., Active optical fibers with wavelength-selective filtering mechanism, method of production and their use, U.S. Pat. No. 8,045,259; T. Taru et al., All solid photonic bandgap fiber, U.S. Pat. No. 8,503,846; J. M. Fini et al., Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier, Optics Letters 31, 2550-2552 (2006); and J. Kim et al., Suppression of stimulated Raman scattering in a high power Yb-doped fiber amplifier using a W-type core with fundamental mode cut-off, Optics Express 14, 5103-5113 (2006), and references therein). Some of these fibers must be bent or coiled to perform as desired (see e.g. J. W. Nicholson et al., Filter fiber for use in Raman lasing applications and techniques for manufacturing same, U.S. Pat. No. 8,428,409; and J. M. Fini and J. W. Nicholson, Optical fiber with distributed bend compensated filtering, U.S. Pat. No. 9,322,989). The concatenation of fibers with different core sizes, including tapered and bent fiber segments, has also been proposed to provide filtering of deleterious light (M. P. Savage-Leuchs, Method and apparatus for optical gain fiber having segments of differing core sizes, U.S. Pat. No. 7,768,700; M. P. Savage-Leuchs, Apparatus and method for optical gain fiber having segments of differing core sizes, U.S. Pat. No. 8,089,689; M. P. Savage-Leuchs, Optical gain fiber having segments of differing core sizes and associated method, U.S. Pat. No. 8,199,399; M. P. Savage-Leuchs, Method and optical gain fiber having segments of differing core size, U.S. Pat. No. 8,345,348; and M. P. Savage-Leuchs, Optical gain fiber having tapered segments of differing core sizes and associated method, U.S. Pat. No. 8,705,166). Self-imaging in a multimode interference filter (MMI) can be used to filter out an undesirable wavelength [V. P. Gapontsev et al., U.S. Pat. No. 7,912,099, supra]. To this end, a segment of multimode fiber is inserted between two single mode fibers. Properly adjusting the length of the multimode fiber segment ensures that a useful wavelength is transmitted with little loss while other wavelengths get attenuated.
Other than performance limitations (see e.g. J. Kim et al., Optics Express 14, (2006), supra; F. Jansen et al., Modeling the inhibition of stimulated Raman scattering in passive and active fibers by lumped spectral filters in high power fiber laser systems, Optics Express 17, 16255-16265 (2009); and D. Nodop et al., Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers, Optics Letters 35, 2982-2984 (2010) for a discussion), a disadvantage of these approaches is their reliance on specific fiber designs. Their implementation requires the insertion of one or multiple segments of fiber within a system. A fiber that is optimal for filtering may not be optimal for other aspects of a system operation. Moreover, these approaches afford little flexibility as the potential performance is predetermined by the fiber design.
Referring to FIG. 3 (PRIOR ART) it is also known to use a uniform period FBG coupled to a circulator to separate useful light from wideband deleterious light. Light enters the circulator through a first port. It then reaches the second port of the circulator where the FBG is connected. Deleterious light at wavelengths outside of the reflectivity spectrum of the FBG is transmitted and leaves the circulator through the second port. Deleterious light at shorter wavelengths than the useful light can also be reflected into cladding modes (not indicated in the figure). Useful light is reflected by the FBG into the fiber core and towards the third port of the circulator. This approach requires a supplementary optical component (the circulator). Transmission through the circulator and a less than 100% reflectivity of the FBG can both induce losses to the useful light. Furthermore, this approach is not well adapted to situations involving high peak powers or high average powers because of the risk of damage to the circulator, either optical or thermal.
FBGs having a chirped period (CFBGs), slanted fringes (SFBGs) or both (CSFBGs) are known in the art of light filtering. Gain flattening in optical fiber communications link has been a major application of CSFBGs, the optical loss of a CSFBG combining with the gain of an amplifier to provide an effective amplification that is uniform over a spectral band of interest [I. Riant and P. Sansonetti, Filter optical waveguide with inclination and linear chirp, U.S. Pat. No. 6,321,008]. CSFBGs have also been used to attenuate light over the spectral band 1520-1565 nm in order to favor amplification over the spectral band 1565-1625 nm in L-band Er-doped fiber amplifiers [R. P. Espindola et al., Article comprising an L-band optical fiber amplifier, U.S. Pat. No. 6,141,142]. The suppression of Raman scattering in optical fibers with lumped filters is discussed in J.-P. Blondel et al., U.S. Pat. No. 6,529,672 (supra) and F. Jansen et al., Optics Express 17, (2009) (supra), both references addressing the optimal positioning of multiple filters along an optical fiber to impede the growth of Raman scattering. Blondel et al stresses the importance of filtering both forward and backward propagating Raman light and the importance of avoiding reflection of light in the fiber core by the lumped filters. Jansen et al proposed using long period gratings (LPG) for filtering. This was followed by an experimental demonstration of the suppression of Raman scattering in a fiber amplifier using LPGs as filters [D. Nodop et al., Optics Letters 35, (2010), (supra)]. Filtering in a LPG and in a SFBG results from coupling light from the core and into the cladding, the difference being that a LPG transmits light into the cladding whereas a SFBG reflects light into it. In both cases, light coupled into the cladding is eventually lost. Gapontsev et al. (supra) discloses the use of SFBGs in a high-power MOPA system to avoid a powerful and potentially destructive reflection of Raman light into a fiber laser oscillator. D. A. V. Kliner and T. S. McComb, Slanted FBG for SRS suppression, US patent application 20160111851 discloses the suppression of Raman scattering with a SFBG that is explicitly chirped.
The suppression of deleterious light inside a laser cavity has been considered as well. J. Liu (Hybrid high power laser to achieve high repetition rate and high pulse energy, US patent application 20060029111) discloses the insertion of FBGs inside a laser cavity, without specifying further the nature of the gratings, to reduce ASE and Raman scattering. H. Po and A. A. Demidov, Multi-wavelength optical fiber, U.S. Pat. No. 7,340,136 discloses the use of LPGs and SFBGs in a Raman laser to suppress the generation of a given Stokes order. In a Raman laser, a cascade of cavities is used to generate light of ever greater wavelength. A first cavity is built to resonate at the wavelength of a pump light. The ensuing high intensity of the pump light leads to the generation of light at a longer wavelength through Raman scattering. This light at a longer wavelength, called the first Stokes order, is used to pump a second cavity designed to resonate at the longer wavelength. The ensuing high intensity at the longer wavelength favors the generation of light at a still longer wavelength through Raman scattering, called the second Stokes order, and so on. H. Po et al. (supra) discloses the insertion of a LPG or SFBG in a cavity to suppress Raman scattering past a desired maximum Stokes order. Even though the origin of the gain sustaining oscillation is different than in a standard laser, the general idea is the same, i.e. the introduction of a filter inside a cavity to impede the generation of undesirable light. Kliner et al. (supra) discloses the insertion of a CSFBG inside a laser cavity to suppress Raman scattering.
To prevent the reflection of light into counter-propagating core modes, SFBG with a pronounced tilt angle of the grating fringes are preferably used [R. Kashyap et al., Wideband gain flattened erbium fibre amplifier using a photosensitive fibre blazed grating, Electronics Letters 29, 154-156 (1993)]. However, as discussed in Riant et al. (supra), a larger tilt angle makes it more difficult to precisely define the spectral response of a CSFBG. The realization of SFBGs producing little reflection in the fiber core has received quite a bit of attention (see e.g. T. A. Strasser and P. S. Westbrook, Article comprising a tilted grating in a single mode waveguide, U.S. Pat. No. 6,427,041; and references found therein). The reduction in reflectivity is achieved by using optical fibers with specially tailored refractive index and photosensitivity profiles (I. Riant et al. (supra); A. Strasser et al., (supra); I. Riant and C. De Barros, Optical waveguide and method for creating an asymmetrical optical filter device, U.S. Pat. No. 7,035,515; S. Ishikawa et al., Optical fiber and fiber grating type filter including the same, U.S. Pat. No. 7,203,399; C. De Barros et al., Photosensitive optical waveguide, U.S. Pat. No. 7,389,022). These profiles are designed to minimize the scattering efficiency between the counter-propagating fundamental core modes. Another approach is to perform the mode coupling in a slightly multimode fiber with a SFBG designed to couple the fundamental core mode with a higher order core mode [C. De Barros et al., Optical filter, U.S. Pat. No. 7,095,924]. The slightly multimode fiber is inserted between two single mode optical fibers that do not support the higher order core mode. A disadvantage of these approaches is again their reliance on specific optical fibers.
There remains a need for efficient filtering of deleterious light in optical fiber devices while alleviating at least some of the drawbacks of the prior art.